Step 2 –

Convert Images to a Stack and Save

With the three separate images open in Image J, choose Image > Stacks > Images to Stack from the menu.

This step creates a single window named "Stack" that shows just one image at a time. Each individual image is referred to as a "slice" in the stack. Keep the Name: Stack and click OK.

Move between the three images using the controller or arrows at the bottom of the window.

Choose File > Save As > TIFF...

Select a place on your computer to store the stack (for instance, in your Documents folder, or in a folder named Pearl River), and give the stack a unique name such as Pearl River.tif.

Step 3 –

Explore the Images

Each image shows an area about 20 km wide by 15 km high in a traditional north is "up" orientation. In the images, vegetation appears in shades of green, water appears in shades of blue, and bare soil appears in shades of tan to light orange. Urbanized areas, covered by buildings and roads, appear off-white to pink.

Examine different parts of the image to identify as many features as you can. Use color and shape clues to help you identify rivers, lakes, roads, rectangular fields, and harbor facilities in the image.

Use the magnifying glass and scrolling tool (hand) to control the image display.

When you are familiar with some of the features, use the animation control bar along the bottom of the stack to step back and forth between the three images to look for changes over time.

(Optional) Each pixel in these images represents a 30-meter square on the ground. If you want to measure features in the images in real-world units, you can select Analyze > Set Scale... and set the Distance in Pixels to 1, the Known Distance to 0.03, and the Unit of Length to km. Once the scale is set, you can select distances or areas on the image and choose Analyze > Measure to see the results. This technique is explained in more detail in the Measuring Distance and Area in Satellite Images chapter.

Step 4 –

Interpret Types of Change

Some differences between the three images are due to normal variation in environmental conditions such as rainfall from year to year. More "permanent" changes such as the appearance of new structures in the harbor and the spread of urban development indicate that humans are changing the area.

What changes can you detect from the first image to the last?

For differences you find, try to decide if the change is due to natural yearly variation or if it is likely to be a human-induced change.

Is there a year that appears to have had less rainfall than the others? What evidence would you cite for your answer?

Over the three images, the colors of vegetation and water varies. These changes appear to be due to normal yearly differences. The appearance of new roads, linear structures in the harbor, and a shift from vegetation to urbanized areas in the later images appear to be evidence of human activity in the region. Based on lower water levels in lakes, 1992 appears to have been drier than the other years.

Step 5 –

Look for Evidence of Land Reclamation

In addition to new structures in the harbor area, some areas of the images show an increase in the area of land. The peninsulas in the center of the images actually grow over time! This unlikely event is the result of land reclamation projects.

To reclaim land, powerful dredging machines gather sand and gravel from the harbor bottom, then heap the sediments into specially prepared enclosures near the shore. Accumulations of the dredged sediments eventually rise above the water level, creating new land.

Look at each of the three slices of the stack carefully by moving through them using the arrows and the magnifiying tool.

Look for areas of land that appear to grow over time and think about the question below.

The rectangular green areas suggest that the new land is used for growing crops.

The practice of moving sediments from river bottoms or seafloors through pipelines back onto land is an obvious example of how humans affect the Earth system–the process is essentially the opposite of erosion. Sand and gravel is sometimes pumped tens of kilometers from where it was deposited by nature on the seafloor. The sediments are then redeposited by the dredging machinery in locations that are uphill from their former location.